The recirculation of exhaust gases, or so-called EGR (Exhaust Gas Recirculation), is a widely known method in which a proportion of the total exhaust gas flow from the engine is recirculated for mixing with intake air to the engine cylinders. This makes it possible to reduce the quantity of nitrogen oxide in the exhaust gases.
This recirculation usually occurs via shunt valves and lines extending on the outside of the engine, from the exhaust gas side to the intake side. In some cases, it is desirable, for reasons of space, to be able to achieve EGR mixing without such arrangements. For this purpose it has been proposed to achieve EGR mixing by using the usual engine inlet and exhaust valves for the return flow of exhaust gases from the engine exhaust manifold to the cylinders, and which is commonly referred to as Internal Exhaust Gas Recirculation (IEGR). The return flow can, in this case, be achieved by an additional opening of a valve, for example the exhaust valve, during the engine operating cycle.
In the case of supercharged diesel engines, however, it may be difficult to supply sufficient excess pressure on the exhaust gas side upstream of the turbocharger to transfer EGR gases to the intake side downstream of the compressor. However, there are pressure pulses on the exhaust gas side, while the inlet pressure is significantly more even. This means that the pressure peaks on the exhaust gas side can be higher than the inlet pressure even though the mean value is lower. If the exhaust valve is opened at such a time of peak pressure during the engine induction stroke, exhaust gases flow back into the cylinder.
The use of a two-position valve clearance, for example, a mechanically adjusted valve clearance combined with a hydraulically adjusted zero-clearance, which can be activated/deactivated according to the engine operating situation, switching between positive engine power output and engine braking (decompression brake), for example, is already known. The additional valve travel which is then activated/deactivated may then be masked by the mechanically adjusted valve clearance, but will appear when zero-clearance is activated. Use of this method may also be considered in order to activate/deactivate an additional valve travel in order to obtain EGR.
The mechanical valve clearance is on the order of 1–3 mm in a typical engine for a heavier road vehicle or truck, for example. The result of this, however, is that the main valve travel needs to have long rise and fall gradients in an order of magnitude at least equal to the mechanical valve clearance. These long gradients are required in order to avoid knocking in the mechanism at the start of the valve travel and to avoid excessively high valve seat landing speeds at the end of the valve travel for both activated and deactivated zero-clearance adjustment. This also means that the main valve travel remains unchanged when the zero-clearance adjustment is activated/deactivated. If the main valve lift has been optimized for operation with EGR activated (zero-clearance activated), for example, the main lift will no longer be optimal when EGR is deactivated (large mechanical clearance), which has a negative effect on the ability of the turbocharger to supply the engine with charge air in critical operating situations. The long gradients also pose a problem in the case of zero-clearance since the exhaust valve travel commences immediately after maximum cylinder pressure has occurred and this results in extremely high stresses in the valve mechanism in order to open the valve in opposition to high cylinder pressure.
It is desirable that apparatus for achieving additional openings of valves should not extend significantly in a longitudinal direction in the space that is available for the engine valve mechanism. For example, the high compression ratios that occur in modern diesel engines mean that the valve mechanism must be designed for very high contact pressure. Furthermore, engines of this type may be equipped with some form of compression brake that needs space for actuators. Apparatus for exhaust gas recirculation (EGR) should therefore not encroach on any compression brake system. The facility for easy engagement and disengagement of the function is also desirable.